Motors are found in many forms. For the purposes of discussion, a three-phase, brushless DC motor (“BLDC motor”) is described. Three-phase brushless DC motors have many uses, among which include both high-speed and low-speed applications. Conventional high-speed applications include spindle motors for computer hard disk drivers, digital versatile disk (DVD) drivers, CD players, tape-drives for video recorders, and blowers for vacuum cleaners. Motors for high-speed applications typically operate in a range from a few thousand rotations per minute (rpm's) to 20,000 rpm's, for example. Low-speed applications include motors for farm and construction equipment, automobile engine cooling fans, HVAC compressors, fuel pumps and the like. Motors for low-speed applications typically operate in a range from less than a few hundred rpm's to a few thousand rpm's, for example. Compared to motors employing brushes, brushless motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.
FIG. 1 illustrates a motor system 20 that includes a sensorless BLDC motor 30, a controller 40, a power-supply Vs and a power-supply control circuit 50. The motor 30 typically includes a permanent magnet rotor and a stator having a number of windings, with the rotor typically housed within the stator. Each winding has a winding tap 32 used to couple the windings to the power-supply control circuit 50. The rotor is permanently magnetized, and turns to align its own magnetic flux with the magnetic flux generated by the windings. The voltage signals at the winding tap 32a, 32b, and 32c are represented as voltages Va, Vb, Vc, respectively.
The power-supply control circuit 50 includes a plurality of pairs of switches Xsa and Xga, Xsb and Xgb, and Xsc and Xbc (collectively “switches X”), one pair for each winding. Each of the switches X is connected to a free end of a winding at the winding tap 32a–32c, and either to the power supply Vs Qr to a ground voltage GND. The switches are typically power transistors such as Mosfets or the like. Reverse biased diodes Dsa, Dga, Dsb, Dgb, Dsc, Dgc (collectively “diode(s) D”) are placed either in parallel with each of these switches or are inherent in the structure of the switches. The diodes D act as power rectifiers, and typically serve to protect the switches X and the motor windings against induced voltages exceeding the supply or ground voltage.
The controller 40 controls and operates the switches X to power the motor 30, typically in a pulse width modulation (PWM) mode. Connecting wires between the controller 40 and the power-supply control circuitry 50 are omitted for clarity. The PWM mode is a mode of operation in which the power is switched on and off at a high frequency in comparison to the angular velocity of the rotor. For example, typical switching frequencies may be in the range of 20 kHz for low-speed applications. In a typical on-off cycle lasting about 50 μS, there may be 40 μS of “on” time followed by 10 μS of “off” time. Given the short duration of off times, residual current still flows through the motor windings so there is virtually no measurable slow down in the angular velocity of the rotor during these periods. Accordingly, because the switches are either full “on” or full “off,” the PWM mode provides a significant power savings over modes in which power is continuously supplied.
The BLDC motor 30 can be represented as having three windings A, B, and C (not shown in FIG. 1), although a larger number of stator windings are often employed with multiple rotor poles. The windings can be connected in a “Wye” configuration, or alternatively in a “Delta” configuration. Typically, in such applications, eight-pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator windings, however, can be analyzed in terms of three “Wye” connected windings, connected in three sets of four windings, each physically separated by 90 degrees.
In operation, the three representative windings A, B and C are energized with a PWM drive signal that causes electromagnetic fields to develop about the windings. The resulting attraction/repulsion between the electromagnetic fields of the windings A, B, and C, and the magnetic fields created by the magnets in the rotor causes the rotor assembly to rotate. The windings are energized in sequences to produce a current path through two windings of the “Wye”, with the third winding left floating (or in tri-state). The sequences are arranged so that as the current paths are changed, or commutated, one of the windings of the current path is switched to float, and the previously floating winding is switched into the current path. The sequences are defined such that when the floating winding is switched into the current path, the direction of the current in the winding that was included in the prior current path is not changed.
The start-up routine for a BLDC motor 30 is handled by the controller 40 and consists of several phases, including pre-positioning the rotor, accelerating the rotor up to a desired speed, and switching to auto-commutated mode, or synchronous mode once the desired speed is reached. In some situations, the BLDC motor 30 may be freewheeling, that is rotating without being driven by the controller 40 before the controller is activated to start and drive the motor. For example, windmilling by an automotive HVAC or engine cooling fan will make the fan motor rotate. Windmilling occurs when air blows across the fan blades, thus causing an unenergized fan motor to rotate. Other situations may occur where the motor 30 is rotating for a reason other than being driven by the controller 40. It is often important to detect the motor 30 rotation direction before beginning the start-up routine. If at the beginning of the start-up routine the motor 30 is turning for a reason other than being driven by the controller 40, the start-up may fail. In addition, the controller 40 or the circuit 50 may be damaged because a high current may flow to the controller 40 if it tries to start the motor 30 while it is rotating in a direction opposite to the desired direction. Further, if the motor 30 is already rotating in a desired direction, the rotation should be preserved rather than losing efficiency by stopping and restarting the motor. Ideally, the controller 40 should “know” the motor 30 rotation in advance of the start-up routine. If the rotation is in the desired direction, then the controller 40 should pick up the speed right away. If the rotation is in the opposite direction, the controller 40 should brake or stop the motor 30 first, and then start the motor in desired direction.
An existing method for determining a pre-start rotation of a motor requires a plurality of sensors installed within the motor structure. Such sensors add components and complexity to a motor system that are used only during motor start up. Therefore, a need exists for an apparatus and a method of determining the rotation direction of a freewheeling motor that requires few or no additions to and little or no alteration of the motor structure.